The invention relates to a cathode including a solid body which comprises metallic constituents (particularly W, Ni, Re, Mo, Pt) and oxidic constituents (such as particularly BaO, CaO, Al.sub.2 O.sub.3, Sc.sub.2 O.sub.3, SrO, ThO.sub.2, La.sub.2 O.sub.3).
Dispenser cathodes include a solid body in the form of a porous metal matrix having more than 70% of metal volume content so that a satisfactory electric conductivity is obtained, as well as an oxide component such as, for example alkaline earth oxide BaO or CaO or 4BaO.CaO.Al.sub.2 O.sub.3 which is present in the pores of the metal matrix or in a dispenser area. When operating such a cathode at 900.degree. to 1000.degree. C., atomic films are produced which consist of the metal(s) (Ba) present in the oxide and atomic oxygen (O) on the metal cathode surface (W) and ensure a low work function. Known cathodes of this type are the I cathode (cf. EP-A 0 333 369) and the scandate cathode (cf. EP-A 0 442 163). Such cathodes have the characteristic features described in the opening paragraph.
At operating temperatures between 900.degree. C. and 1000.degree. C. saturation current densities of between 10 and 150 A/cm.sup.2 are achieved. Such cathodes require relatively high heating temperatures which limit the lifetime due to destruction of the W heating coil.
Oxide cathodes (cf. EP-A 0 395 157) have a relatively thick porous oxide layer of alkaline earth oxides (for example, Bao.SrO) and further oxide dopants (for example, Sc.sub.2 O.sub.3, Eu.sub.2 O.sub.3) on a metal support such as nickel. They can be used at substantially lower operating temperatures of approximately 730.degree. to 850.degree. C. with emission current densities of 10 to 50 A/cm.sup.2, but only in the .mu.sec range. Because of the low electric conductivity of the oxide components, the permanent load capacity is limited to 1-3 A/cm.sup.2.